The present invention relates to a steering apparatus for a vehicle, which is constructed to perform steering by transmitting rotation of a motor driven according to an operation of a steering member, such as a steering wheel, to a steering shaft in a steering mechanism and moving the steering shaft in the axial direction.
A vehicle is steered by transmitting an operation of a steering member executed by a driver (in general, turning of a steering wheel) to a steering shaft in a steering mechanism (for example, a rack shaft in a rack/pinion type steering apparatus) and pushing and pulling vehicle wheels for steering (in general, the left and right front wheels) by the movement of the steering shaft in the axial direction to turn the wheels.
As a steering apparatus for a vehicle for performing such steering, an electric power steering apparatus, which is constructed to assist steering performed by mechanical power transmission from a steering member by driving a steering assist motor provided in a part of the steering mechanism, according to an operation of the steering member, and transmitting the rotation force of the motor to a steering shaft, is widely used in practical applications. Besides, a separation type steering apparatus, that is, a so-called steer-by-wire steering apparatus, which comprises a steering mechanism mechanically separated from a steering member and a steering motor provided in a part of the steering mechanism, and performs steering by only the rotation of the motor driven and controlled according to an operation of the steering member, is under development.
A steering apparatus for a vehicle comprising a steering assist motor or a steering motor as described above requires movement converting means for converting rotation of such a motor into movement in an axial direction of the steering shaft. Widely used examples of the movement converting means include a ball screw mechanism which is constructed by coaxially screwing a cylindrical nut member rotatably supported in a cylindrical housing supporting a steering shaft onto a screw groove formed over an appropriate length in the outer surface of the steering shaft, rotates the nut member by power transmission from a motor, and moves the steering shaft in an axial direction by the spiral movement of the screw groove according to the rotation, and particularly a ball screw mechanism which is constructed by screwing a screw groove in the outer circumference of a steering shaft and a nut member through a number of balls, and can highly efficiently transmit power with rolling movement of the balls (for example, U.S. Pat. No. 6,155,376).
A steering apparatus for a vehicle with the above-described screw mechanism is constructed by dividing a cylindrical housing supporting the steering shaft into a first housing and a second housing connected coaxially, supporting a nut member inside the connection section of these housings, and interactively connecting the nut member to a motor provided on the outside of the first housing or second housing so as to rotate the nut member by power transmission from the motor.
Here, reaction forces act on the nut member screwed onto the steering shaft in both directions along the axial direction with the above-mentioned movement conversion. It is therefore necessary to support the nut member by a thrust bearing capable of carrying thrust loads in both directions, and the steering apparatus for a vehicle disclosed in U.S. Pat. No. 6,155,376 adopts a support structure in which the nut member is thrust supported by a bearing held and fixed in the connection section between the first housing and second housing.
However, in this support structure, the thrust loads applied in both directions to the nut member as described above are applied to one of the first and second housings in a direction apart from the other. Therefore, great connection strength is required for the connection section of the first and second housings, and it is necessary to increase the thickness of connection flanges and the size and number of bolts for tightening the flanges, and thus this structure suffers from the drawbacks that the freedom of design is decreased and the weight of the product is increased.
Besides, in the case where the housing for supporting the steering shaft is constructed in a divided manner by the first and second housings, a fixing structure in which fixing means (fixing bracket) for fastening to a vehicle body is provided for each housing, and each fixing means is individually positioned with respect to a fixing seat on the vehicle body side and fixed by tightening a bolt, is adopted In many cases, the fixing means of the first housing is integrally constructed and the fixing means of the second housing is constructed in a separate member to enable changing of position in a circumferential direction and an axial direction, so that positioning with respect to the fixing seat on the vehicle body side is readily performed.
However, when such a fixing structure is adopted, the fixation of the second housing comprising fixing means in a separate member becomes unstable due to the function of thrust forces repeatedly applied from the nut member in the inside, and this instability may influence steering performed by the movement of the sheering shaft and deteriorate steering feeling.